1. Field of the Invention
The present invention relates to a semiconductor light emitting device.
2. Description of the Prior Art
A semiconductor light emitting device such as a semiconductor laser device or a light emitting diode employing a group III nitride semiconductor (hereinafter referred to as a nitride based semiconductor) such as GaN, GaInN, AlGaN or AlGaInN is expected for application to a light emitting device emitting light over the visible to ultraviolet regions. Such a semiconductor light emitting device is formed on a (0001) plane of a substrate of sapphire, silicon carbide or the like by MOVCD (metal-organic chemical vapor deposition) or MBE (molecular beam epitaxy).
In a GaN based semiconductor light emitting device formed by successively stacking an n-type semiconductor layer, an emission layer and a p-type semiconductor layer on a substrate, an n-type current blocking layer is generally formed in the p-type semiconductor layer. This current blocking layer performs transverse mode control of the semiconductor light emitting device.
FIG. 7 is a typical sectional view showing an exemplary conventional GaN based semiconductor laser device 200.
In the semiconductor laser device 200 shown in FIG. 7, a buffer layer 82 of undoped AlGaN, an n-contact layer 83 of n-GaN, an n-light cladding layer 84 of n-AlGaN, an n-light guide layer 85 of n-GaN, an emission layer 86, a p-cap layer 87 of p-AlGaN, a p-light guide layer 88 of p-GaN and an n-current blocking layer 89 having an opening are successively formed on a sapphire substrate 81. A p-light cladding layer 90 of p-AlGaN is formed in the opening of the n-current blocking layer 89. A p-contact layer 91 of p-GaN is formed on the p-light cladding layer 90 and the n-current blocking layer 89.
Partial regions of the layers from the p-contact layer 91 to the n-contact layer 83 are etched so that an n-type electrode 50 is formed on the exposed part of the n-contact layer 83. A p-type electrode 51 is formed on the p-contact layer 91.
In the semiconductor laser device 200, the n-current blocking layer 89 narrows current injected from the p-type electrode 51. Thus, the opening of the n-current blocking layer 89 defines a current injection region while an emission part is formed in the region of the emission layer 86 located under the current injection region.
The material for the current blocking layer 89 may be n-AlGaN, n-InGaN or the like.
When prepared from n-AlGaN, the n-current blocking layer 89 has a small refractive index due to Al contained therein. The region of the emission layer 86 located under the n-current blocking layer 89 having a small refractive index exhibits a smaller effective refractive index as compared with the region of the emission layer 86 located under the opening. Light is horizontally confined in the emission layer 86 due to such distribution of the refractive index. This device structure confining light by the difference in refractive index is referred to as a real refractive index guided structure.
When prepared from n-InGaN, on the other hand, the n-current blocking layer 89 having a smaller band gap than the emission layer 86 absorbs light of a higher mode generated in the emission layer 86. Thus, light is concentrated to the region of the emission layer 86 located under the opening of the current blocking layer 89, and horizontally confined in the emission layer 86. This device structure confining light by light absorption is referred to as a loss guided structure.
As described above, transverse mode control is performed in the semiconductor laser device 200 due to the current narrowing in the n-current blocking layer 89 and confinement of light in the emission layer 86.
In the semiconductor laser device 200 having the n-current blocking layer 89 of n-AlGaN, the electron concentration in the n-current blocking layer 89 is generally extremely increased to 1019 to 1020 cmxe2x88x923, thereby suppressing leakage of current in the n-current blocking layer 89 and reducing current which does not contribute to the laser oscillation.
In order to increase the effect of transverse mode control in the semiconductor laser device 200, the Al composition of the n-current blocking layer 89 made of n-AlGaN is preferably increased. When the Al composition is increased, the refractive index of the n-current blocking layer 89 is further reduced thereby increasing the difference in refractive index of the emission layer 86 along the horizontal direction. Thus, light is effectively confined.
When having a large Al composition, the n-current blocking layer 89 exhibits a lattice constant smaller than that of the n-contact layer 83, to generate an electric field (piezoelectric field) as a result of piezoelectric effect caused by tensile strain. However, electrons of a high concentration are injected into the n-current blocking layer 89 and hence the piezoelectric field is reduced by movement of the electrons. Thus, the energy band is inhibited from bending caused by the piezoelectric effect.
FIG. 8 is a model diagram showing the energy band structure of the semiconductor laser device 200 having the n-current blocking layer 89 of n-AlGaN in a section taken along the line Bxe2x80x94B in FIG. 7. Referring to FIG. 8, positive bias applied between the p-type electrode 51 and the n-type electrode 50 is zero.
As shown in FIG. 8, the energy band of the n-current blocking layer 89 is flat since the high-concentration electrons suppress the piezoelectric effect in the n-current blocking layer 89.
When applying positive bias between the p-type electrode 51 and the n-type electrode 50 of the semiconductor laser device 200, a quasi Fermi level lowers on the side of the p-contact layer 91 and rises on the side of the n-contact layer 83 as shown by arrows in FIG. 8. Thus, the energy band of the n-current blocking layer 89 is inclined toward the upper right. When applying higher positive bias, the inclination of the energy band of the n-current blocking layer 89 is so increased that holes can move from the p-contact layer 91 to the p-light guide layer 88 through the n-current blocking layer 89 due to tunnel effect if the n-current blocking layer 89 has a small thickness. Consequently, current which does not contribute to the laser oscillation is increased. Further, holes falling to the level in the n-current blocking layer 89 cause recombination different from desirable emission, to increase the current which does not contribute to the laser oscillation.
Such current that does not, contribute to the laser oscillation can be suppressed by increasing the thickness of the n-current blocking layer 89. When thickly growing the n-current blocking layer 89 having a high Al composition, however, cracking results from strain caused by lattice incommensurateness with the n-contact layer 83. Therefore, it is difficult to increase the thickness of the n-current blocking layer 89 of n-AlGaN.
Transverse mode control can be performed with the n-current blocking layer 89 whose thickness is small to some extent. When the n-current blocking layer 89 has a small thickness, however, it is difficult to suppress leakage of current caused by the aforementioned tunnel effect and hence the current which does not contribute to the laser oscillation is increased. Thus, operating current and threshold current of the semiconductor laser device 200 are increased to reduce the luminous efficiency.
Also in a semiconductor laser device of the loss guided structure having an n-current blocking layer of n-InGaN, current which does not contribute to the laser oscillation is increased if the n-current blocking layer has a small thickness and hence operating current and threshold current of the semiconductor laser device are increased to reduce the luminous efficiency similarly to the semiconductor laser device 200 having the current blocking layer 89 of n-AlGaN.
An object of the present invention is to provide a semiconductor light emitting device having low operating current and low threshold current with high luminous efficiency, which can reduce of current which does not contribute to the laser oscillation.
The semiconductor light emitting device according to the present invention comprises a first conductivity type first semiconductor layer, a second conductivity type second semiconductor layer having a current injection region, an emission layer provided between the first semiconductor layer and the second semiconductor layer, into which current is injected through the current injection region due to application of positive bias between the first semiconductor layer and the second semiconductor layer, and a current blocking layer, provided in the second semiconductor layer except the current injection region, containing an electric field reverse to the positive bias.
In this semiconductor light emitting device, the current blocking layer contains the electric field reverse to the positive bias, whereby the positive bias applied between the first semiconductor layer and the second semiconductor layer reduces the electric field contained in the current blocking layer. Thus, a potential gradient in the current blocking layer disappears or decreases so that no current leakage results from tunnel effect but current is reliably blocked in the current blocking layer. Thus, current which does not contribute to the laser oscillation decreases and operating current as well as threshold current are reduced in the semiconductor light emitting device, whereby a semiconductor light emitting device having high luminous efficiency is obtained.
The current blocking layer may contain the electric field reverse to the positive bias by piezoelectric effect. In this case, the electric field is generated in the current blocking layer by the piezoelectric effect. The electric field reverse to the positive bias can be contained in the current blocking layer by setting the composition of the current blocking layer so that the electric field by the piezoelectric effect is reverse to the positive bias.
The current blocking layer may have strain accompanied by generation of the electric field reverse to the positive bias. In this case, the electric field is generated in the current blocking layer due to the strain of the current blocking layer. The electric field reverse to the positive bias can be contained in the current blocking layer by setting the composition of the current blocking layer so that the electric field generated by the strain is reverse to the positive bias.
The second semiconductor layer may include a first conductivity type carrier concentration layer, stacked on the current blocking layer, having a carrier concentration higher than that of the current blocking layer.
When the first conductivity type high carrier concentration layer is provided between the current blocking layer and the second conductivity type second semiconductor layer, a depletion layer is formed in the second semiconductor layer having a lower carrier concentration than the high carrier concentration layer.
In the semiconductor light remitting device having no depletion layer formed in the current blocking layer, carriers in the second semiconductor layer are prevented from diffusing into the current blocking layer thorough the depletion layer. Thus, current can be further reliably prevented in the current blocking layer.
The current blocking layer may be made of a nitride based semiconductor containing at least one of gallium, aluminum, indium, thallium and boron, and the first semiconductor layer, the emission layer and the second semiconductor layer may be made of a nitride based semiconductor containing at least one of gallium, aluminum, indium, thallium and boron. In such a nitride based semiconductor, the electric field generated by the piezoelectric effect remarkably appears.
The current blocking layer preferably has a crystal growth surface prepared from a (0001) plane of the nitride based semiconductor. In the current blocking layer having such a crystal growth surface, the piezoelectric effect is maximized so that the current blocking layer contains the electric field reverse to the positive bias.
The carrier concentration of the current blocking layer is preferably lower than 1xc3x971019 cmxe2x88x923.
If the carrier concentration of the current blocking layer is higher than 1xc3x971019 cmxe2x88x923, the electric field generated by the piezoelectric effect is reduced and hence a potential gradient is caused in the current blocking layer upon application of positive bias. When the carrier concentration of the current blocking layer is lower than 1xc3x971019 cmxe2x88x923, the current blocking layer can contain the electric field reverse to the positive bias by the piezoelectric effect, whereby the potential gradient in the current blocking layer decreases or disappears upon application of positive bias.
The refractive index of the current blocking layer may be smaller than the refractive index of the current injection region. In this case, the effective refractive index of the region of the emission layer located under the current blocking layer is reduced as compared with the effective refractive index of the region of the emission layer located under the current injection region. Light is horizontally confined in the emission layer due to such difference in refractive index in the emission layer. Thus, a semiconductor light emitting device having a real refractive index guided structure is implemented.
The current blocking layer may be made of a nitride based semiconductor containing aluminum and gallium and the surface of the current blocking layer may be terminated with nitrogen while the aluminum composition thereof may be greater than 0.1. The semiconductor light emitting device may further comprise a sapphire substrate, and the first semiconductor layer, the emission layer, the second semiconductor layer and the current blocking layer may be formed on the sapphire substrate.
In this case, the refractive index of the current blocking layer containing aluminum can be reduced. Thus, a semiconductor light emitting device having a real refractive index guided structure is implemented.
Further, the aluminum composition of the current blocking layer is greater than 0.1 and the surface of the current blocking layer is terminated with nitrogen, whereby the current blocking layer contains the electric field reverse to the positive bias.
Particularly in a nitride based semiconductor containing aluminum and gallium formed on a sapphire substrate, piezoelectric effect is caused by strain. Thus, the current blocking layer made of the nitride based semiconductor containing aluminum and gallium formed on the sapphire substrate contains the electric field reverse to the positive bias.
The current blocking layer may made of a nitride based semiconductor containing aluminum and gallium and the surface f the current blocking layer may be terminated with gallium and aluminum, while the aluminum composition may be less than 0.1. The semiconductor light emitting device may further comprise a sapphire substrate, and the first semiconductor layer, the emission layer, the second semiconductor layer and the current blocking layer may be formed on the sapphire substrate.
In this case, the refractive index of the current blocking layer containing aluminum can be reduced. Thus, a semiconductor light emitting device having a real refractive index guided structure is implemented.
Further, the aluminum composition of the current blocking layer is less than 0.1 and the surface of the current blocking layer is terminated with gallium and aluminum, i.e., group III elements, whereby the current blocking layer contains the electric field reverse to the positive bias.
Particularly in a nitride based semiconductor containing aluminum and gallium formed on a sapphire substrate, piezoelectric effect is caused by strain. Thus, the current blocking layer made of the nitride based semiconductor containing aluminum and gallium formed on the sapphire substrate contains the electric field reverse to the positive bias.
The band gap of the current blocking layer may be not more than the band gap of the emission layer.
In this case, the current blocking layer absorbs light of a higher mode generated in the emission layer. Thus, light is concentrated to the region of the emission layer located under the current injection region and horizontally confined in the emission layer. Therefore, a semiconductor light emitting device having a loss guided structure is implemented.
The current blocking layer may be made of a nitride based semiconductor containing indium and gallium. The semiconductor light emitting device may further comprise a silicon carbide substrate, and the first semiconductor layer, the emission layer, the second semiconductor layer and the current blocking layer may be formed on the silicon carbide substrate.
In this case, the band gap of the current blocking layer containing indium can be rendered smaller than the band gap of the emission layer. Thus, a semiconductor light emitting device having a loss guided structure is implemented.
Particularly in a nitride based semiconductor containing indium and gallium formed on a silicon carbide substrate, piezoelectric effect is caused by strain. Thus, the current blocking layer made of the nitride based semiconductor containing indium and gallium formed on the silicon carbide substrate contains the electric field reverse to the positive bias.
The first semiconductor layer may include a first conductivity type cladding layer, and the second semiconductor layer may include a second conductivity type cladding layer. In this case, a semiconductor light emitting device having high luminous efficiency is implemented.